Oscillator with beat frequency stabilization



Dec. 9, 1947.

w. `L.. CARLSON oscILLATR WITH BEAT FREQUENCY STABILIZATION Filed oct. 21, 1943` 4 sheeis-snee 2 Ill.

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V WendZL L. (/arbson AT TO'QNE K Patented Dec. 9, 1947 OSCILLATOR WITH BEAT FREQUENCY STABILIZATIDN Wendell L. Carlson, Princeton, N. J., assgnor to Radio Corporation of of Delaware America, a corporation Application October 21, 1943, Serial No. 507,126

(Cl. Z50-36) 9 Claims.

This invention relates to multiple tube oscillators and more particularly to systems for stabilizing the frequency of an oscillator without the aid of crystal control.

The causes of instability in the frequency of oscillators are primarily temperature and humidity changes, load changes, and voltage fluctuations. The effect of changing load, and the effect of capacity and inductance varying with temperature, may be minimized by good design. |Thus, the remaining causes of frequency instability are all concerned with the oscillator tube itself. For example, a fluctuation in supply voltages will change the tube resistance and tube capacities which naturally aiect the frequency. A further undesirable frequency change may result when the oscillator tube is replaced, if the new tube has different inter-element capacities. Also, the frequency may vary with time as the tube warms up.

Since changes in tube constants contribute so largely t frequency instability, it seems desirable to devise circuits in which the frequency will be more nearly independent of the tube constants. The most direct way of approaching this condition is to couple the tube weakly to the tuned circuit or to use a low L/C ratio. The phrase to couple weakly to is used to signify tapping the tube across a `low impedance portion of the tuned circuit rather than a loosening of the coupling between fixed reactive circuit elements..

The degree t0 which the coupling can be reduced is limited by the mutual conductance of the oscillator tube. Obviously, greater frequency stability would result if tubes of higher mutual conductance were available. since the effect of the oscillator tube on the tuned circuit could be more nearly eliminated by the expedient of reduced coupling. If, however, the ,gain of the oscillator tube could be supplemented by means of an amplifier then very loose coupling to the frequency determining circuit could be used. Unfortunately, the use of looser coupling results in no increase in frequency stability if phase shifts occur elsewhere in the circuit, because such phase shifts must be cancelled by a contrary phase shift in the tuned circuit, which can only be obtained by a change in frequency. Thus, it is evident that the use of an amplifier at the` same frequency has little or no advantage.

The advantages of amplification with minimum phase shift can be obtained by use of a low frequency ampliiier. Two heterodyne detectors are required to accomplish the two necessary fre- 2 quency conversions. AFor heterodyning, the same local oscillator is used fin both detectors. The frequency of this oscillator differs from the desired frequency by the frequency of the amplier.

It is an object of my invention to Yprovide an oscillator tunable over a Wide range of frequencies and having great frequency stability.

Another object of my invention is to provide an oscillator having a multiple tube circuit which operates on the heterodyne principle for producing interstage frequency compensation.

Still another object of my invention is to provide a multiple tube oscillator having feedback means between two of its stages and an automatic volume control circuit arrangement in addition to other means for frequency stabilization.

Another object of my invention is to provide a multiple tube oscillator of the type indicated in the preceding paragraph and also characterized by the provision -of an automatic frequency control device associated with the oscillatory circuit of one of the tubes.

The foregoing and other objects of my invention will be more clearly understood in view of the following description taken in connection with the accompanying drawings, in which:

Fig. l shows a multiple tube oscillator circuit arrangement embodying the fundamental features of my invention;

Fig. 2 shows another embodiment of my invention in which features of automatic Volume control are added;

' Fig. 3 shows Vstill another embodiment of my invention in which automatic frequency control means are added to the elements in combination as shown in Fig. 2; and

Fig. 4 shows a chart of frequency deviation vin relation to percentagesof decrease in the anode and screen supply voltages from a predetermined value.

Referring first to Fig. 1, I show therein an oscillatory circuit which comprises vedischarge tubes l, 2, 3, d, and 5 and associated circuit connections. oscillations of a desired frequency are generated in the `tube 3 in the anode circuit of which is a feedbackinductance 6 loosely coupled to the inductance 'l in a parallel tuned circuitof high frequency stability. The parallel tuning is accomplished by means of a variable capacitor 8. The coupling between the inductances 6 and l is preferably made quite loose in order to minimize the phase shift of the oscillations in the feedback circuit. For the same reason, loose coupling is provided between the tuned circuit l, 8

3 and control grid 9 in the converter tube I. This condition is obtained by tapping the coil 1 at a point quite close to its grounded terminal.

Tube I is also controlled through its grid IIIy by oscillations generated in the tube 4 at a relatively lower frequency. Tube I, therefore, operates to heterodyne the higher and lower frequencies and to deliver an output at a difference frequency to which its output circuit is tuned. The anode II is, therefore, connected to the plus terminal of a power supply through a resistive impedance I2 and the inductance I3 of a tuned circuit. Parallel tuning is obtained by means of the adjustable capacitor I4.

The inductance I3 constitutes a transformer primary, the secondary I5 of which forms part of another parallel tuned circuit inclusive of adjustable capacitor I6. The tuned circuit I5,

I6 is coupled to the input circuit of tube 2 across formenthe secondary 2I of which is paralleltuned by capacitor 22.y The tuned circuit 2|, 22 is disposed between a first grid 23 in tube 3 and ground. Tube 3 is also controlled by means of its grid 24 in accordance with oscillations from the generator 4 but by way of a buffer amplifier tube 5. These are high frequency oscillations of a frequency less than the desired output frequency by the intermediate frequency whichv is delivered through tube 2. Tube 3, therefore, actsas a second converter for heterodyning the frequencies impressed upon its. grids 23. and 24. The output from tube 3 Vis both a sum and adifference frequency, but the sum frequency alone is utilized in the .feedback circuit which includes the inductance 6. Anode potential is supplied to tube 3 through resistors 25 and 26. y

Tube 4 represents any suitable oscillator, but for illustrative purposes, it is here shown as of Vthe well-known Hartleytype having its cathode connected to an intermediate point on the parallel-tuned inductance 21. Inductance 21 is disposed in parallel-tuned relation to the adjustable capacitor 28. -Anode potential is supplied through resistor 25 and a choke 29. The control grid 30;

is maintained at ground potential for alternating current, being coupled to ground across capacitor 3l. A grid leak resistor 32 is provided in connection between the grid and the cathode 33.

The buffer stage previously mentioned as including tube 5has its control grid connected to a point of low alternating potential on the inductance 21. The screen grid of'tube 5 is supplied with a suitable potential across resistor 34. Anode potential is also supplied across re-sistor 35. The output circuit of tube 5 includes cathode resistor 36 which is grounded at the lower end. The anode is coupled across capacitor 31 to grid 24 in the converter tube 3. The control potentials for grid 24 are developed across a resistor `ilwbich is connected between one electrode of capacitor 31 and ground. Y

A cathode resistor 4B is shown between the cathode 4I in tube I and ground. The second and fourth grids 42 in tube I both act as screens and are supplied with screen grid potential across resistor 43. Grids 42 are also coupled to ground across capacitor 44.

In tube 2, the control grid 45 is connected to ground across grid vleak resistor 46. The screen grid 41 is supplied with suitable D. C. potential across resistor 48 and this grid is also coupled to ground across capacitor 49.

In tube 3, cathode 50 is connected to ground across a cathode resistor 5I, while its second and fourth grids 52, serving as screens, are supplied with suitable D. C. potential across resistor 53 and these grids 52 are coupled to ground across capacitor 54.

The operation of the circuit arrangement of Fig. 1 will be more readily understood by explaining the same in reference to a specific selection of frequencies to be used in the different portions of the circuit. These frequencies, however, are to be understood as chosen merely for purposes of illustration and the invention itself is not restricted to the use of any particular frequency. If the frequency of the oscillator 4 is set at 9650 kc., the tank circuit 1, 8 may be tuned to 10,000 kc. Heterodyning of these two frequencies produces a difference-frequency output from tube I of 350 kc. CircuitsI3, I4 and I5, I6 are very broadly tuned to 350 kc. Likewise circuits IS, I9 and 2|, 22 are also broadly tuned to 350 kc. The broad tuning is for the purpose of minimizing phase shifting of the signal passing through the low frequency amplifier. Higher amplification can be obtained with a minimum of phase shift by amplifying at a substantially lower frequency than the desired output frequency.

A modification of the low frequency amplifier may consist of employing single resonant circuits instead of coupled circuits. Thus, for example, circuit I 5, I6 may be omitted andthe anode end of the tank circuit I4, I3 may be coupled directly to condenser I1. A similar change may be made in the circuit coupling tubes 2 and 3. In this case circuit I3, I4 is preferably tuned to about 400 kc;

and circuit I8, I9 is tuned to about 300 kc. in the illustrative form as described. This arrangement minimizes phase variations in the low frequency current passing through the amplifier. Such variations are unavoidably accompanied by frequency drift and are Vdue to small frequency variations inthe oscillator associated with tube 4. At the main frequency of the amplifier, the coupling circuits, therefore, act like nearly pure reactances so that a change in mutual conductance or'in tube capacity or a change in intermediate frequency produces very little phase shift. In an embodiment which has actually been constructed according to my invention. the extent to which the output frequency was found to be independent of the amplifier frequency is evidenced by the fact that a change in the local oscillator frequency of 28 kilocycles in tube 4 produced a change of only two kilocycles in the output frequency from tubek 3.

To summarize the operation of Fig. 1, the desired output frequency is derived from the second converter tube 3 by coupling its output circuit to a utilization device through capacitor 39. Tube 3 derives its control from the intermediate frequency amplifier 2 (which delivers a low intermediate frequency to the grid 23), and from oscillations produced by the oscillator 4, which are transferred through the buffer amplifier stage 5 to the grid 24. Heterodyning takes place in tube 3 in such manner as to deliver a sum-frequency to the feedback circuit, the inductance 6 of which is coupled to the tuned resonant tank circuit 1, 8. The latter is a circuit which is isolated so far as possible from other components of the entire circuit arrangement in order that its frequency stability may be maintained. Its loose coupling with the grid 9 in tube l, however, enables this tube to act as a first converter, where oscillations of the frequency generated by tube 4 are delivered to grid lil. The intermediate frequency which, in fact, is a difference-frequency` output from tube i is amplied in tube 2 and utilized in the second converter tube 3.y

Referring now to Fig. 2,v I show therein a modified circuit arrangement which, however, follows generally the principles set forth in respect to the circuit arrangement of Fig, 1. Additional features are provided which include an automatic volume control circuit for assisting in the maintenance of frequency stability.

A conventional oscillator circuit 55 may be considered as a substitute for the electronic oscillator i in Fig. 1. The output from oscillator 55 is coupled across the capacitor 56 to one end of a load resistor 51, the other end being grounded. The A. C. potential developed across resistor 5l is applied to control grids 58 and 59 in two buffer amplifier tubes 59 and 6| respectively. These tubes are preferably of a conventional pentode type having three grids, a cathode, and an anode. Anode potential is supplied from the direct current power supply (so labelled) across the resistor 82. Screen grid potential is also supplied from the same source across resistor 53. These connections are the same for both tubes 60 and 8 I.

Output potential from tube 60 is coupled across capacitor 5f! to a third grid l0 in a first conk verter tube 55 corresponding to tube I in Figl l. This coupling develops control potentials across the gridI leak resisto-r 98. `Tube E5 is preferably of the heptode type wherein the rst and third grids are used for control purposes, the second and fourth grids are screens and the fth grid, connected to the cathode, serves as a suppressor. The grid 9, nearest the cathode, is preferably controlled by A. C. potentials derived from the parallel-resonant circuit consisting of inductance 'l and adjustable capacitor 8. One end of this parallel-resonant circuit is grounded, and a tap 55 r' for the utilization of oscillatory potentials is made near the grounded end f the resonant circuit in order to supply a necessary control voltage across the grid leak resistor l for application to the grid 9 in tube G5, A coupling for this circuit is provided by capacitor 58. The screen grids 42 in tube 55 are maintained at suitable D. C. potential derived from the .power supply and fed across resisto-rs 99 and l0. Through resistor 69 D, C, potential is also carried to the anode I! in tube 85, where a tuning inductance il is series-connected to the anode circuit. Capacitor l2 serves to bypass alternating current potentials from the screens 52 to ground. Capacitor I3 bypasses to ground any alternating potentials that may appear at the junction between resistor 69 and inductance 1l.

An intermediate frequency amplifier stage cornprises tube ifi which occupies the position of tube 2 in Fig. l. Tube M is preferably of the pentode type. Its screen grid 'i is fed with D. C. potential across resistor 55, and A. C. potentials ap-V pearing on the screen i? are bypassed to ground across capacitor 59. The cathode of tube 74 is provided with a cathode resistor 82, in parallel with which is a bypass capacitor 83. The output circuit for tube 'M includes a parallel-resonant tank consisting of inductance 84 and tuning condenser 85. point of zero potential swing on the resonant circuit 84, 85 to ground. The control grid 'l5 receives potentials from the output circuit of tube 65 by virtue of the coupling capacitor 16. Control potentials are developed across the grid leak resistor 11, the latter being connected to a suitable point on an AVC circuit, so labelled. Both of the tubes 65 and 'I4 are subject to automatic volume control where the AVC potential is derived from the rectification of a component of output energy from a seco-nd converter tube 78. This tube occupies the positicn of tube 3 in Fig. 1. It is, however, preferably of the heptode type wherein the rst grid 23 receives its control potentials by virtue of a coupling capacitor 19 connected to the anode in tube 'M and also to Va grid leak resistor 89.

A suitable negative bias is applied to grid 23 by means of a grid biasing source 8 I, the positive terminal of which is grounded, while its negative terminal is connected to one end of the grid leak resistor 80.

Tube 'I8 is preferably provided with a suppressor grid 81, but in other respects, its construction includes the same electrodes as are shown in tube 3 (Fig. 1). The output circuit for tube I8 may be traced from the anode 88 thro-ugh the feedback coil 5 and thence through resistors 26 and 2,5 tothe positive terminal of the power supply. The negative terminal of this power supply is connected to the grounded cathode 50. Screen grid potential is applied to the screens 52 across resistors 25 and 53.

AA parallel-resonant circuit consisting of inductance 89 and capacitor 90 is disposed in shunt with resistor 25, this resonant circuit being tuned to the frequency which is to be utilized as output from the entire circuit arrangement. The output to a utilization device is preferably coupled to the anode 88 across capacitor 39. The voltage swing developed in the resonant circuit 89, 90 appears at the upper end as shown in the diagram, the lower end being maintained at A. C. ground potential by virtue of the bypass capacitor 9|, one terminal of which is grounded.

AVC potentials are developed by means of a rectifier tube 92 to the anode of which an output component derived from the feedback circuit is applied across capacitor 93 and resistor 94. The cathode in tube 92 is maintained at ground potential, The time constant value of elements 93 and 94 is made such as to provide a suitable delay action in the AVC circuit. This circuit includes series resistors 95, 96, and 91, where resistors 95 and 96 are common to the input circuits of tubes 65 and 14, and resistor 91 feeds AVC potential through resistor 51 exclusively to the grid 9 in tube B5.

The basic operation of the circuit arrangement shown in Fig. 2 may be understood by reference to the foregoing explanation applicable to Fig. 1. The same frequencies may also be considered by way of illustration. That is to say, if the oscillator 55 generates a frequency of 9,650 kc. and if the resonant circuit l, 8 is tuned to 10,000 kc., then a difference-frequency of 350 kc. is obtained from the heterodyne action in tube 55. The tuned input circuit for the amplifier tube 14 is preferably adjusted to a value of substantially 490 kc., whereas its output circuit is preferably tuned to substantially 300 kc, A desired b and- Bypass condenser 85 couples a Y the second converter tube 73.

pass characteristic of the amplifier tube 'I4 is thus obtained for favoring the amplification of the difference-frequency at 350 kc. which is ap.- plied as one componentV of the input energy for The other frequency applied to this tube at the third grid 24 is derived indirectly from the oscillator 55, controlling, as it does, the buffer amplifier tube 6I. The control potentials from the anode of tube 6| are coupled across capacitor 99 directly to grid 2li in tube 18.

The AVC circuit operates upon the rst converter tube grid 9 and the intermediate frequency tube grid 'I5 in such manner that no grid current is allowed to ow in the second converter tube grid circuit including grid 23 and its grid leak resistor 80.

In adjusting the parameters of the circuit arrangement of Fig. 2 for optimum operating conditions, it is advisable to obtain a Q which is as high as possible in the output tuned circuit. The achievement of this aim results in greater stability of the resulting output frequency since it facilitates the use of very loose coupling with respect to the feedback coil B and also to the input circuit of tube 65. In a specific embodiment of the invention which was constructed in accordance with Fig. 2, the Q o-f the tuned circuit 8 was designed to be approximately 150, being limited to a readily obtainable value at 10 megacycles when the diameter of the coil was xed at substantially 1/2 inch and the coil itself Was wound on a standard RCA Styrol form. The advantages of minimizing the grid current in such places as the second converter tube I8 by means of auto-matic bias control may be Well appreciated by those acquainted with the prior art in connection with frequency stabilization of oscillators. It is also a well-known fact that appreciable improvement in frequency stability results fro-m the reduction of the grid current to a small value. Consequently, in carrying out my invention, still further frequency stability is accomplished by the elimination of grid current. Hence, the grid bias is preferably to be set in accordance with the oscillation amplitude by means of an automatic volume control amplifier.

In order to increase further the frequency stability in a multi-stage oscillator, I have found it advantageous to incorporate automatic frequency control with the double heterodyne principle as heretofore described. This may be accomplished, for example, by utilizing the system which is illustratively shown in the circuit diagram of Fig. 3.

Referring to Fig. 3, I show therein the essential components of a double heterodyne oseillator having automatic volume and frequency control. The circuit arrangement is quite similar to that which is shown in Fig. 2 but with a discriminator stage and a reactance tube added for the purpose of exercising frequency control over the oscillator which is t deliver a lower-than-wanted frequency for heterodyning purposes.

The resonant circuit l, 8 is used in the same relation to a first converter tube 55 as is shown in Fig. 2. The same oscillator 55 is shown in both figures. Other elements bear corresponding reference numbers in the two gures and are similarly disposed in the circuit arrangement. These elements include the I. F. amplifier tube 14, the second converter tube '|8, and associated circuits.

VThe output circuit from the second converter "tube 'I8 may be traced from its anode 88 through the feedback coil 6 and thence to one terminal of a parallel-tuned circuit comprising a capacitor |00 and an inductance IGI. This inductance forms the primary winding of a transformer |02, the secondary |03 of which is the inductance element of another parallel-tuned circuit wherein the capacitance is composed of two series-connected condensers I 04. The intercoupled resonant circuits |50, |9| and |03, |04 are both tuned to the I. F. frequency which is applied to the control grid 23 in tube 18. In shunt with the tuned circuit |00, |0| is a resistor |05 and another resistor |05 is in shunt with the resonant circuit |03, |04,

Anode potential for the second converter tube I8 is supplied from the positive terminal of the direct current source through resistor |05 in parallel with the inductance I0 I, and `thence through the feedback coil 6 to the anode 88.

The terminals of the resonant circuit |03, |04 are connected to the two anodes respectively of a twin diode tube |51. The two cathodes of this twin diode are separate but cathode |08 is connected to ground while cathode I 09 is connected to an automatic frequency control circuit for supplying suitable control bias to the rst grid I |0 in a reactance tube III. The cathodes |08 and |09 are interconnected through resistors ||2 and are also intercoupled by series capacitors I3.

Interposed between the cathode |09 in the twin rdiode |01 and the input circuit for the reactance tube ||I is a grid bias adjusting device comprising an independent direct current source I I4, the terminals of which are connected respectively to the two ends of a potentiometer I I5. An adjustable tap on this potentiometer is connected to the grid ||0 of the reactance tube I Il through a grid leak resistor H5.

The reactance tube possesses a screen grid ||'I on which suitable screen grid potential is impressed through resistor IIS. The anode H9 is connected to the anode in the oscillator tube 55.

Alternating current control potentials are supplied to the grid I I0 in the reactance tube by way of resistor |20 and capacitor |2|. These potentials are derived from the resonant circuit 21, 28 which is associated with the oscillator tube 55.

In general, the circuit parameters for the different tube stages are conventional and require no further description in view of the detailed descriptions which have been supplied above in reference to Figs. 1 and 2. The discriminator circuit arrangement which includes the transformer |02 and its associated tuning devices and also the twin diode rectifier I0? consitute a discriminator circuit arrangement of conventional type. A similar discriminator is described in my Patent No. 2,243,414 which issued May 27, 1941. Other discriminators are well kno-wn in the art and may rthe frequency control circuit for the oscillator 55.

To summarize, however, it will be seen that if the desired output frequency is to be that to which the Vresonant circuit l, 8 is tuned, and if the frequency of output from the oscillator 55 is somewhat lower than the wanted frequency, then the first converter tube 65 will serve to deliver an intermediate frequency, say of 350 kc., as in the illustration given with respect to Figs. 1 and 2. In the second vconverter stage which includes the tube 'I8 and its circuit components, the output from the I. F. amplifier 'M is applied to grid 23 while the output from the oscillator 55 is applied to the third grid 24. The output circuit which includes the feedback coil 6, therefore, has three principal frequency components, namely, the frequency of the oscillator 55, the I. F. frequency, and the sum of these two frequencies, the latter being that to which the resonant circuit 1, B is tuned. Thus the sum-frequency is also available for utilization purposes as by coupling across the capacitor 39.

It will be recalled that the resonant circuit which includes the windings of the transformer |02 are both tuned to the I. F. frequency. The

` phase shift in the intermediate frequency amplifier is preferably minimized by the employment of suitable resistance loading in the associated circuits. To obtain a small amount of coupling to the input circuit of the second converter while maintaining the high Q which is desired, an overall gain of approximately '7,000 is preferably chosen.

The AVC voltage derived from the discriminator in Fig. 3 is developed between ground and the mid-tap on the resistive element |05. This Voltage also appears as a potential drop across the upper half of resistor I I2 and is negative with respect to ground since the cathode |08 is maintained at ground potential. The automatic frequency control voltage is developed positively with respect to ground in accordance with the rectification which takes place in the lower half of the twin diode tube |01, the cathode |09 of which becomes more and more positive as the amplitude of the oscillations increases. This tends, therefore, to apply a more positive bias to the grid I Ill in the reactance tube i thereby to reduce the impedance of the reactance tube l As a result the anode voltage in tubes and 55 is reduced to compensate for such frequency increase in the output from oscillator 55 as may be attributed to a` voltage increase in the D. C. supply source.

The slopes of the reactance tube and discrimi- -nator characteristics are capable of such determination that a 10G-cycle change in intermediate frequency produces a 10-kilocycle change in a l-megacycle local oscillator frequency, i. e., a control ratio of 100. The measurement of the frequency change in the local oscillator produced by the addition of a small shunt capacitor to the oscillator tank, conditions being compared with and without the feedback circuit present, shows that the local oscillator frequency may be changed in the ratio 1/50 by the presence of the feedback circuit.

Fig. 4 shows the degree of frequency stabilization which is possible of attainment in carrying out my invention. The horizontal scale shows percentages of voltage decrease from normal, in respect to anode, screen grid, and filament voltages. The frequency deviation in cycles from a nominal value of 10 megacycles is indicated on the vertical scale. The full-line curves show the coordination of voltage vs. frequency deviation when utilizing automatic volume control and also automatic frequency control as described in reference to Fig. 3. The curves shown in broken lines give corresponding values of frequency de- '15 10 viation when automatic volume control alone is used in accordancewith the embodiment shown and described in reference to Fig. 2. `It is apparent from observation of Fig. 4 that a high degree of frequency stability is obtainable in multi-tube oscillators when operating in a frequency range such as to include .10 megacycles, for example, by means of multi-tube feedback circuits which employ either the double heterodyne scheme with automatic volume control alone or by means of such a system with automatic frequency control added.

While my invention is described in the foregoing and illustrated in the drawings so as to enable anyone skilled in the art to follow my teachings and to adopt advantageous practices in the' design of stabilized multi-tube oscillators, I wish it to be understood that the invention itself is by no means limitedto the specific form and construction of the circuit arrangements so described and illustrated.

I claim:

1. In an oscillation generator having a plurality of intercoupled stages, including two mixer tube stages and an intervening difference-frequency amplifier stage, a resonant circuit disposed in loosely coupled relation to the input of the first of said mixer tube stages, a source of oscillations the frequency of which when heterodyned with oscillations from said resonant circuit produces said difference-frequency, means for feeding energy from said oscillation source to both of said mixer tube stages thereby to cause heterodyning in one case with the frequency of said resonant circuit, and in the other case with output from said amplifier stage, and a feedback circuit originating at said second mixer tube stage and loosely coupled to said resonant circuit, said feedback circuit being arranged to control the generated oscillations by a sum-frequency component of output fromv the second of said mixer tube stages.

2,. A circuit arrangement according to claim l and including parallel-tuned input androutput circuits for said amplifier stage, said paralleltuned circuits being resonant in one case to a higher frequency and in the other case to a lower frequency than said difference-frequency.

3. A circuit arrangement according to claim 1 and including a volume control device operative upon the gainin the first mixer tube stage and in said amplifier stage ,inlresponse to rectified output energy from said second mixer tube stage.

4. A circuit arrangement according to ,claim 1 and including means for causing the output from said oscillation source to be frequency-controlled by output from said second mixer tube stage.

5. In a multi-tube oscillation generator, an electronic oscillator tuned to a frequency below that which is wanted for utilization, a resonant circuit tuned to that wanted frequency, a cascade arrangement of two converter tube stages and an intervening amplifier tube stage, the first of said converter tube stages having input circuit means for mixing energy components derived from said oscillator and from said resonant circuit, said amplifier stage having a filter bandpass characteristic which accepts the differencefrequency derived from heterodyning output energy from said electronic oscillator with oscillations from said resonant circuit, means effective in the second of said converter tube stages for mixing two frequencies, namely, that of said electronic oscillator and said difference-frequency passed by the amplifier stage, thereby to deliver a sum-frequency substantially corresponding to the frequency of said resonant circuit, and feedback means loosely coupling the output from the second converter tube stage to said resonant circuit.

6. A multi-tube oscillation generator comprising a rst converter stage, a tuned resonant circuit coupled to the input side of said converter stage, an external source of oscillations the output from which is lfed to said converter stage and is of a different frequency in relation to that to which said resonant circuit is tuned, an output circuit for said converter stage having a lterpass characteristic which accepts a differencesaid component of heterodyned energy, an out-A put circuit for said amplier stage having a filter-pass characteristic which partially overlaps the frequency band of said converter stage output circuit, whereby said diierence-frequency component also controls a subsequent stage, said subsequent stage being constituted as a second converter having control electrodes connected respectively to the output side of said amplier stage and to said external source, said second converter having an output circuit which includes an inductance loosely coupled to said tuned resonant circuit, and means for utilizing in said second converter stage an amplied energy component of said external source of oscillations, whereby the useful output from said second converter is of a frequency corresponding substantially to that of said resonant circuit.

V7. A multi-tube oscillation generator comprising a rst and a second converter tube stage, an amplifier stage interposed between and in cascaded relation to said converter tube stages, said amplifier stage having input and output circuits 'which are tuned to frequencies one slightly above and one slightly below a difference-frequency component 'of output energy derived from the first converter, means including a resonant circuit and anV external source of oscillations having diferent frequency'characteristics for producing a heterodyne action in said first converter stage such as to cause the delivery of said differencefrequency output energy tofsaid ampliier stage, means including a'buffer amplier interposed between the output circuit of said external source and the input to said second converter for mixing therein the frequency components of said external source and of saiddilTerence-frequency passed by the irst said amplifier stage; a feedback device in the output circuit of said second converter coupled to said resonant circuit, and automatic volume control means controlled by rectitied energy derived from said second converter and applied to the control grids of said rst converter and the rst said amplier respectively.

8. A multi-tube oscillation generator comprising a rst and a second converter tube stage, an amplifier stage interposed between and in cascaded relation to said converter tube stages, said amplier stage having input and output circuits which are broadly tuned to a relatively low difference-frequency component of output energy derived from the rst converter, means including a resonant circuit and an external source of oscillations having dilferent frequency characteristics for producing a heterodyne action in said rst converter stage such as to cause the delivery of said diierence-frequency output energy to said amplifier stage, means including a buffer amplifier interposed between the output circuit of said external source and the input to said second converter for mixing therein the frequency components of said external source and of said difference-frequency passed by the rst said amplier stage, a feedback device in the output circuit of said second converter coupled to said resonant circuit, and automatic volume control means controlled by rectied energy derived from said second converterfand applied to the control grids of said rst converter and the first said amplifier respectively.

9. An oscillation generator according to claim 8 and including automatic frequency control means under control of output energy from said second converter stage and effectively applied to the stabilization of the output frequency of said external source. f o

WENDELL L. CARLSON.

REFERENCES CITED The following references are of record in the le of rthis patent:

UNITED STATES PATENTS Number K Name Date s 2,228,323 4Mortley Jan. 14. 1941 1,978,818 YRoosenstein Oct. 30,1934

OTHER REFERENCES Electronics for May 1940, pages 32 and 33. (Copy in Division 51 and in Library.) 

